In general, in a direct-injection-type internal combustion engine (in particular, a direct-injection-type diesel engine), fuel is injected into a combustion chamber at the near compression top dead center in a state in which the temperature and pressure of the cylinder interior gas have been increased through compression. Consequently, atomized fuel (fuel mist) diffuses within the combustion chamber, and successively autoignites and burns (i.e., diffusion combustion, diesel combustion).
In such diffusion combustion, the fuel concentration of the fuel mist (i.e., excess air ratio λ or equivalent ratio φ) becomes nonuniform in the course of diffusion of the fuel mist. Thus, in a region in which the equivalent ratio φ is near 1 (stoichiometric region), NOx is produced because of abrupt chemical reaction (heat generation), and in a region in which the equivalent ratio φ is greater than 1 (rich region), smoke, particulate matter, or the like (hereinafter collectively referred to as “PM”) is produced because of deficiency of oxygen.
Conventionally, there have been known various techniques for individually reducing the generation quantities of NOx and PM. However, a trade-off unavoidably arises between the generation quantities of NOX and PM, so that when the generation quantity of one of these substances is reduced, the generation quantity of the other substance increases. Therefore, presently, simultaneous reduction of both the generation quantity of NOx and that of PM is very difficult.
In view of the foregoing, in recent years, there has been proposed a combustion scheme in which fuel is injected into a combustion chamber earlier than the near compression top dead center (in a state in which the cylinder interior gas has relatively low temperature and pressure), and the quantity of EGR gas (EGR ratio) is increased so as to render the ignition delay time relatively long, to thereby cause the premixed gas mixture diffused substantially uniformly within the combustion chamber to autoignite at the near compression top dead center (hereinafter referred to as “premixed combustion” or “PCCI (premixed-charge compression ignition) combustion”).
In such PCCI combustion, since the premixed gas mixture having sufficiently and uniformly diffused within the combustion chamber ignites in a generally uniform lean state (state in which the equivalent ratio φ is less than 1), both NOx and PM are hardly generated. As a result, both the generation quantity of NOx and that of PM greatly decrease as compared with the case of the above-mentioned diffusion combustion.
However, when the entirety of the premixed gas mixture having been widely dispersed within the combustion chamber in a generally uniform lean state ignites simultaneously, a relatively loud explosion sound (noise) tends to be generated. This tendency becomes remarkable when the delay time of ignition of the premixed gas mixture is excessively short (accordingly, when autoignition of the premixed gas mixture starts too early). Further, since the premixed gas mixture is dispersed in a generally uniform lean state, misfire tends to occur easily. This tendency becomes remarkable when the delay time of ignition of the premixed gas mixture is excessively long (accordingly, when autoignition of the premixed gas mixture starts too late). In other words, in PCCI combustion, the timing at which autoignition of the premixed gas mixture starts (or the ignition delay time) must be accurately controlled such that the start timing coincides with a predetermined timing or falls within a predetermined range. For this accurate control, the autoignition start time of the premixed gas mixture must be estimated accurately.
In view of the above, a control apparatus for a diesel engine disclosed in Japanese Patent Application Laid-Open (kokai) No. H11-148412 is designed, on the basis of the fact that the ignition delay time is strongly influenced by the compression end temperature of cylinder interior gas, so as to obtain the compression end temperature of the cylinder interior gas, and to estimate the ignition delay time (accordingly, the autoignition start time of the premixed gas mixture) from only the obtained compression end temperature. Further, under the assumption that in the compression stroke the state of the cylinder interior gas causes a polytropic change (adiabatic change), the compression end temperature of the cylinder interior gas is obtained from the cylinder interior gas temperature at the compression start time (time of closure of the intake valve) and a formula representing a general polytropic change.
Incidentally, in the above-described PCCI combustion, a portion of the premixed gas mixture is known to cause a low-temperature oxidation reaction (called “cool flame” in the present specification and the claims) prior to autoignition of the premixed gas mixture (accordingly, generation of a hot flame). When such a cool flame is produced, the temperature of the cylinder interior gas may change greatly because of influence of the quantity of heat generated by the cool flame. Accordingly, the autoignition start time of the premixed gas mixture is considered to greatly depend on the heat generation quantity of the cool flame.
However, the apparatus described in the patent publication estimates the ignition delay time (accordingly, the autoignition start time of the premixed gas mixture) from only the compression end temperature of the cylinder interior gas which is obtained under the assumption that the state of the cylinder interior gas causes a polytropic change in the compression stroke, without consideration of the influence of the heat generation quantity of the cool flame. Accordingly, the conventional apparatus cannot accurately estimate the autoignition start time of the premixed gas mixture.